Updating an existing dataset using a diffgram

[GoogleGroups]

It seems like this should be easy, but it has proven to be quite
painful.

I have an existing dataset, which I used the GetChanges and WriteXML
functions upon to write out a DiffGram to a local XML file. I want to
use this same dataset (in another instance of the application) and the
DiffGram to display the changes in the DataGridView as they existed
after the user made his/her change.

In English:
A user will open the application, which displays data in a grid, make
their changes and hit save. This will save only the changes (in a
diffgram) to a local file. They will then come in the next day and
make a few more changes. I would like the application to grab the
original data and merge their previous changes into the original data
and display that on the grid.

I've used both ReadXML as well as Merge, but ReadXML only appends the
rows in the DiffGram to the end, and Merge doesn't seem to do anything
but toss the DiffGram square into the garbage.

Here is my code:

Dim dsList As New DataSet
Dim dsChanges As New DataSet

mySqlDataAdapte r.AcceptChanges DuringFill = False
mySqlDataAdapte r.Fill(dsList)

If File.Exists(str Changes) AndAlso
File.ReadAllTex t(strChanges).L ength 0 Then

' ReadXml on the original dataset just adds it to the
end
'dsList.ReadXml (strChanges, XmlReadMode.Dif fGram)
'.Tables(0).

dsChanges.ReadX ml(strChanges, XmlReadMode.Dif fGram)

dsList.Merge(ds Changes, True)

End If

grdResults.Data Source = dsList.Tables(0 )

Please help!

[GoogleGroups]

The solution is as painful as I had hoped...

Turns out that you need some form of unique ID (primary key) to match
the data, so I had to create a new column in the original dataset as an
identity field. You then need to set the primary key in both datasets
so that it knows how to match them. In addition, instead of merging
the changes back onto the original list, you must merge the list onto
the changes dataset. In order to do that, you need to grab the schema
of the original data set and set it to the changes schema. Confused?
Hopefully the code will be clearer than the description:

Dim mySqlDataAdapte r As SqlDataAdapter = New
SqlDataAdapter( cmdList)

Dim dsList As New DataSet
Dim dsChanges As New DataSet

' Populate the list from the DB
mySqlDataAdapte r.Fill(dsList)

' If the changes file exists (in diffgram format)
If File.Exists(str Changes) AndAlso
File.ReadAllTex t(strChanges).L ength 0 Then

' Write out the schema from the list
File.WriteAllTe xt("D:\temp\Sch ema.xml",
dsList.GetXmlSc hema(), System.Text.Enc oding.Unicode)

' Set the schema of the changes dataset
dsChanges.ReadX mlSchema("D:\te mp\Schema.xml")

' Populate the changes dataset
dsChanges.ReadX ml(strChanges, XmlReadMode.Dif fGram)

' Set the primary key values for both datasets
dsList.Tables(0 ).PrimaryKey = New DataColumn()
{dsList.Tables( 0).Columns("ID" )}
dsChanges.Table s(0).PrimaryKey = New DataColumn()
{dsChanges.Tabl es(0).Columns(" ID")}

' Merge the list onto the changes, not the other way
around
dsChanges.Merge (dsList, True)

' Set the datasource of the grid
grdResults.Data Source = dsChanges.Table s(0)
Else
' Set the datasource of the grid
grdResults.Data Source = dsList.Tables(0 )
End If